Heart failure is a debilitating disease in which abnormal function of the heart leads can result in blood flow that is insufficient to fulfill the needs of the tissues and organs of the body. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract. Often, the ventricles do not adequately eject or fill with blood between heartbeats and the valves regulating blood flow become leaky, allowing regurgitation or back-flow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness and the inability to carry out daily tasks may result. Not all heart failure patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive. As heart failure progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds muscle causing the ventricles (particularly the left ventricle) to grow in volume in an attempt to pump more blood with each heartbeat. This places a still higher demand on the heart's oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output. A particularly severe form of heart failure is congestive heart failure (CHF) wherein the weak pumping of the heart leads to build-up of fluids (i.e. congestives) in the lungs and other organs and tissues.
Heart failure has been classified by the New York Heart Association (NYHA) into four classes of progressively worsening symptoms and diminished exercise capacity. Class I corresponds to no limitation wherein ordinary physical activity does not cause undue fatigue, shortness of breath, or palpitation. Class II corresponds to slight limitation of physical activity wherein such patients are comfortable at rest, but wherein ordinary physical activity results in fatigue, shortness of breath, palpitations or angina. Class III corresponds to a marked limitation of physical activity wherein, although patients are comfortable at rest, even less than ordinary activity will lead to symptoms. Class IV corresponds to inability to carry on any physical activity without discomfort, wherein symptoms of heart failure are present even at rest and where increased discomfort is experienced with any physical activity.
The current standard treatment for heart failure is typically centered on medical treatment using angiotensin converting enzyme (ACE) inhibitors, diuretics, beta-blockade, and digitalis. Cardiac resynchronization therapy (CRT) may also be employed, if a bi-ventricular pacing device is implanted. Briefly, CRT seeks to normalize asynchronous cardiac electrical activation and resultant asynchronous contractions associated with CHF by delivering synchronized pacing stimulus to both ventricles. The stimulus is synchronized so as to improve overall cardiac function. This may have the additional beneficial effect of reducing the susceptibility to life-threatening tachyarrhythmias. CRT and related therapies are discussed in, for example, U.S. Pat. No. 6,643,546 to Mathis et al., entitled “Multi-Electrode Apparatus and Method for Treatment of Congestive Heart Failure”; U.S. Pat. No. 6,628,988 to Kramer et al., entitled “Apparatus and Method for Reversal of Myocardial Remodeling With Electrical Stimulation”; and U.S. Pat. No. 6,512,952 to Stahmann et al., entitled “Method and Apparatus for Maintaining Synchronized Pacing.”
In view of the potential severity of heart failure, it is highly desirable to detect its onset within a patient and to track its progression so that appropriate therapy can be provided. Many patients suffering heart failure already have pacemakers or ICDs implanted therein or are candidates for such devices. Accordingly, it is desirable to provide such devices with the capability to automatically detect and track heart failure. Heretofore, a number of attempts have been made to provide for monitoring of physiological parameters associated with heart failure using implantable cardiac devices in conjunction with physiological sensors. End diastolic pressure (EDP) has been found to be indicative of heart failure and various techniques have been developed for detecting heart failure based on EDP or related pressure parameters. See, for example, U.S. Pat. No. 6,438,408 to Mulligan et al., entitled “Implantable Medical Device for Monitoring Congestive Heart Failure”; U.S. Pat. No. 6,277,078 to Porat et al., entitled “System and method for monitoring a parameter associated with the performance of a heart”; U.S. Pat. No. 6,666,826 to Salo et al., entitled “Method and Apparatus for Measuring Left Ventricular Pressure”; U.S. Pat. No. 6,580,946 to Struble, entitled “Pressure-Modulated Rate-Responsive Cardiac Pacing”; and U.S. Patent Application 2002/0120200 of Brockway et al., entitled “Devices, Systems and Methods For Endocardial Pressure Measurement.”
However, it can be difficult to reliably measure EDP using an implanted system. Moreover, heart failure can begin to occur without a change in EDP. Indeed, severe cardiac depression can occur in the presence of “normal” blood pressures. Accordingly, alternatives to pressure-based heart failure detection techniques have been proposed. In particular, measurements of cardiac output have been found to be effective in detecting and tracking heart failure, as cardiac output usually decreases with heart failure. See, for example, U.S. Pat. No. 6,314,323 to Ekwall, entitled “Heart Stimulator Determining Cardiac Output, by Measuring The Systolic Pressure, for Controlling The Stimulation” and U.S. Pat. No. 6,572,557 to Tchou et al., entitled “System and Method for Monitoring Progression of Cardiac Disease State Using Physiologic Sensors.” Cardiac output measurements are particularly advantageous since they can be used as an indicator of acute decompensation before such decompensation is reflected in blood pressure elevation, and hence can provide an early warning of the onset of heart failure.
One promising technique for monitoring cardiac output is thermal dilution wherein a portion of blood passing through the heart is heated and the resulting blood temperature profile is detected downstream using a thermistor. The downstream temperature profile is then evaluated to estimate cardiac output based on conservation of energy principles. See, for example, U.S. Pat. No. 5,174,299 to Nelson entitled “Thermocouple-Based Blood Flow Sensor”; U.S. Pat. No. 5,217,019 to Hughes, entitled “Apparatus and Method for Continuously Monitoring Cardiac Output”; U.S. Pat. No. 5,285,796 also to Hughes, entitled “Method for Continuously Monitoring Cardiac Output”; and U.S. Pat. No. 5,682,899 to Nashef et al., entitled “Apparatus and Method for Continuous Cardiac Output Monitoring.”
Heretofore, however, thermal dilution techniques for measuring cardiac output are not practical given current technology. A significant problem with thermal dilution techniques is that a considerable amount of energy is required to heat the blood, thus depleting the energy reserves of the implanted device, which are preferably reserved for delivering defibrillation shocks or the like.
Accordingly, it would be highly desirable to provide improved techniques for use by an implantable device for detecting cardiac output via thermal dilution, which do not consume significant energy resources of the implanted device. It is also desirable to provide techniques for detecting and tracking heart failure based on cardiac output detected using the improved thermal dilution techniques. These and other objectives were achieved by the invention set forth in the parent application. Briefly, an implantable device was described that is capable of performing thermal dilution analysis of the cardiac output of a patient using power delivered from an external source. By using power from an external source, the implantable device conserves its own power resources for other purposes, such as for delivering pacing or defibrillation therapy. In one example, an external programmer or bedside monitor provides power via a hand-held power delivery wand. The wand is placed over the chest of the patient in the vicinity of a subcutaneous power reception coil and power is transferred thereto using electromagnetic induction. The power is then routed to a heating coil implanted in the right atrium, which heats blood as it passes through the right atrium. A resulting downstream blood temperature profile is detected using a thermistor implanted in the pulmonary artery. The cardiac output of the patient is then estimated by analyzing the temperature profile. The techniques of the parent application are also described herein below.
Although the invention of the parent application is effective, room for improvement remains. In particular, it would be desirable to provide for delivery of power from an external source directly to the heating element of the right atrium, so as to eliminate the need for a separate power reception coil and circuitry for relaying power from the receiving coil to the heating coil. It would also be desirable to eliminate the need for transmission of power via electromagnetic induction since electromagnetic induction signals can potentially interfere with the operation of other electronic devices, such as those commonly found in hospitals, clinics and the like. It is to these ends that the invention of the present application is primarily directed.